1. Technical Field of the Invention
The present invention relates to cellular radio communications, and more particularly, to a method and apparatus for converting between a cellular bit stream and a wireline modem waveform.
2. Description of Related Art
Wireline networks are currently defining new services based upon Integrated Service Digital Network's (ISDN's) flexible standards for delivering direct digital interfaces for subscribers in a variety of bit rates. These variety of bit rates cannot be directly mapped to wireless services without an as yet undefined compression technique because of the premium placed on efficient radio spectrum utilization.
One proposed method of providing maximum flexibility for future mapping between wireline waveforms and cellular bit streams involves defining a transparent mode in which all data bits of, for example, the IS54 format totaling 13 KB/S are mapped to a PSTN subscriber using a suitable wireline modem waveform for greater than 13 KB/S, for example, 19.2 KB/S. The IS54 channel and speech coding and decoding could be remotely located at the mobile switching center (MSC) or even a private address branch exchange (PABX).
However, this method only involves mapping "hard" bit (class I speech bit) decisions from the IS54 signal. By mapping only "hard" bit decisions from IS54 to wireline, the "soft" bit information that is valuable in performing an alternative error correction decoding for data would be discarded. The "soft" bit information is accessible only in the vicinity of the base station receiver's demodulator, unless the bit rate between the receiver and the wireline is increased substantially to preserve the soft information. This suggests that channel decoding of the convolutionally coded speech bits should be performed at the transceiver for both voice and data nodes in order to benefit from the "soft" bit information to achieve best performance. Thus, a method for converting between a cellular bit stream and a wireline modem waveform is needed which preserve the benefits of "soft" error correction decoding for both voice and data.